Analysis of T cell differentiation has been greatly aided by two findings: (a) that conversion of pro-thymocytes to thymocytes can be induced in a brief assay in vitro, and (b) that conversion of thymocytes to functional T cells of different subclasses is associated with the surface expression of particular sets of Ly components that correspond with particular immune functions. The athymic nude mouse is a natural model of T cell deprivation with which the therapeutic effect of T-cell inducers can be studied in vivo, because all known immunologic defects of the nude mouse can be attributed to failure of T cell differentiative steps subsequent to the pro-thymocyte stage. Thymopoietin selectively induces pro-thymocytes. Ubiquitin non-selectively induces pro-thymocytes, pro-B cells, and doubtless other commited cells. We propose to test thymopoietin and ubiquitin, synthetic polypeptides based on the sequences of the parent molecules, and synthetic analogues (as available) for differentiative effects on T and B cell precursors in newborn and adult nude mice, by the criteria of immunogenetic surface markers, especially the Ly set, and by tests of immune function. We ask whether thymopoietin can initiate functions that ubiquitin cannot, whether synthetic fragments are more or less effective than the parent molecules, whether the effects of non-specific induction by ubiquitin may counteract effects that thymopoietin may have when given alone, and whether the nude mouse model will yield the same answers as the newborn-thymectomized and adult-thymectomized mouse models being studied by Goldstein. The ultimate aim is therapeutic control of immune-responsiveness.